1. Technical Field of the Invention
The present invention relates to an apparatus and a process for screening a fibre suspension such as papermaking stock in a pressurized screen especially in connection with pumping said liquid. The invention also relates to the use of the apparatus in a process for the production of paper or board.
The invention relates principally to the removal of impurities from fibre slurries used in the pulp and paper industries. The invention presents an improvement of commonly used pressurized screens, by which the loss of good material together with rejects is minimized so that the cleaning process becomes more efficient, compact and energy efficient and the apparatus is made less complicated. The invention further allows the integration of the pumping, cleaning and stock distribution functions with the screening, whereby the papermaking process is further simplified.
2. Prior Art
In known screens, presented among others in U.S. Pat. No. 3,363,759, a fibre slurry is fed into a space limited at least in one direction by a screen plate through which the fibre suspension is forced, and which prevents fibre bundles and other bigger particles from passing through the screen together with the accepted fraction, i.e. the accept. Due to the mechanism of screening, also a part of the good fibres stop at the screen plate, forming a fibre mat which gradually thickens and prevents the flow through the screen. According to the above mentioned US Patent the fibres collecting at the screen plate are brought back into the suspension by means of turbulence generating bumps in the surface of a rotor.
The temporary hold of fibres at the screen plate also makes the liquid component of the suspension pass the screen faster than the fibres, which causes the suspension being screened to become gradually more concentrated in the screening zone. This gradually decreases the functionality of the screening, until the screening has to be interrupted in order to prevent excessive concentration and a resulting clogging of the screen.
Removing the fraction which is not to pass the screen, i.e. the reject, from the screen, also requires a sufficient volume flow so that the flow speed in reject pipes can be kept sufficiently high, and the separation of solids and consequent clogging of the piping can be avoided.
Due to these phenomena a significant portion, typically 5 to 30%, of the good fibres screened are rejected when screening in a conventional screen. In order to recover these fibres, the reject is re-diluted and cleaned in a second cleaning stage in another screen, the reject of which can be cleaned in a third screen, and so on. The last cleaning stage, normally, comprises an open screen, from which the reject can be removed at a high consistency.
Screening in a multiple of screens is obviously disadvantageous in view of the need for space, investment, energy consumption, cleanliness and also in view of process controllability.
In previously used open screens it was common practice to dilute the goods to be screened in the screening zone. Internal dilution has been attempted, also for pressurized screens and the resulting difficulties have been resolved in various ways. U.S. Pat. No. 3,437,204 avoids stratifications and poor mixing, when diluting through a rotor, by feeding dilution water through a screen plate. EP Patent Application 0233517, aiming at the washing of recycled wastepaper, also introduces water through an opening in the screen plate. Both these solutions lead to consistency differences at the accept side, which would require further equalizing before use in a paper machine. There are also other drawbacks, such as the fast draining of dilution water and consequently an excessive need for such water, complicated screen maintenance, etc.
A problem with most pressurized screens of the prior art is that the rotors have to generate pressure pulses for keeping the screen plate open and permitting the fibres to pass. These pressure pulses are conducted further in a papermaking process, and are a source of disturbance of the forming process, which requires a very stable and pulse-free flow.
SE Patent 331629 describes a pressurized screen with a screen-drum rotating in a cylindrical housing. The screen-drum may be adjusted from a central position to an eccentric one for causing turbulence at the screen. This provides a mechanically complicated solution to the pulsation problem, but leaves the problem related to the thickening of the goods to be screened unresolved.
FI Patent 46414 also describes a screen with a screen-drum rotating in a cylindrical housing. The screen-drum is rotated so as to provide Couette flow in a suspension being screened. Pulsation means are provided for generating pulsations in the flow of particles through the screen. The screen housing may be provided with slits for feeding dilution water into the annular screening chamber for preventing the reject layer collected on the screen from whirling up along the housing.
In conventional screens, the increasing consistency of the goods to be screened cause differences also in the consistency of the accept at different areas of the screening plate. These differences are known to cause consistency stratifications in the accept flow, which stratifications in turn may cause irregularities in a forming paper web.
Most papermachine wet ends involve centrifugal cleaning of the papermaking stock, by means of hydrocyclones. The hydrocyclones typically require an operating pressure differential of 100 . . . 200 kPa and operate in a multiple of hydrocyclones, whereby typically 200 . . . 400 kJ of energy is consumed for every cubic meter of cleaned stock. The hydrocyclones further constitute a considerable investment, and they require continual maintenance.
A generic problem when forming paper or board is the uniform distribution of stock in the cross machine direction and management of profile irregularities. In the same applicant's copending Patent Application WO 93/23609 a multiple of distribution pipes having essentially identical lengths and flow resistances lead directly from the screen to the headbox, thus regulating the stock distribution profile.
The traditional papermachine wet end involves huge volumes of circulating backwater and many feedback loops for secondary cleaning and screening stages. The same applicant's copending Patent Application WO 93/23612 (FI Patent 89728) provides a novel solution to the problems regarding controllability and cleanliness in a papermaking process. Said solution essentially eliminates the huge water volumes and the feedbacks in a paper machine short circulation.